Here is a video interview from the "Serenity" Los Angeles Premiere held at Universal City Cinemas, September 22, 2005.

Summer Glau talks about her role in Serenity and some things that she has in common with her character River Tam.
Screencaps from the video can be found here: Interviews > 2005 > 'Serenity' Premiere at Universal Studios

Look at how expressive Summers face is. An average person doesn't use half that many expressions IMO( thought the situation isn't average either as people don't usually give interviews but the difference wouldn't be that much).

This proves again that Summers fantastic acting abilities are mostly the results of born talent, its in her blood. Summer said that she could relate to River because she was as lonely and lost as River when she was a teenager. I think thought that there is one another thing which makes Summer similar to River, namely the prodigy part

She also looks gorgeous in that outfit of course. I'm usually more fan of her simpler, more Cameron-ish outfits but this one is fantastic too. That beautiful flower in her beautiful hair paired with that smile makes her as bright as the sun when you look at her.